Photolithography tools are primarily used for the manufacture of integrated circuits (IC) or other micro devices. With a photolithography tool, a reticle pattern can be imaged onto a wafer coated with photoresist. In the photolithography tool, a projection objective is employed to expose the reticle pattern so that the latter is transferred into the photoresist. As a key component of the photolithography tool, a wafer edge protection device plays an important role in edge protection for wafers coated with negative photoresist during exposure.
There are two types of photoresist, also known as resist or sensitizer: positive photoresist, in which the portion exposed to light becomes soluble to the photoresist developer; and negative photoresist, in which the unexposed portion is dissolved by the photoresist developer. Wafer edge protection devices are designed for photolithography processes using negative photoresist.
In conventional wafer edge protection devices, a protective ring is utilized to shield a wafer edge so as to protect the wafer edge from being exposed. Such a protective ring is a centrally hollow, peripherally solid optical shield. Such a wafer edge protection device includes a mechanical assembly for transport of the protective ring and a control module. When a wafer stage of the photolithography tool with a wafer carried thereon is moved to a location under the wafer edge protection device for protective ring transfer, the mechanical assembly will transfer the protective ring onto the wafer stage or pick it up therefrom under the control of the control module. Referring to FIGS. 3a and 4a, this conventional wafer edge protection device does not have any speed control device, and the mechanical assembly is enabled to move vertically under the effect of the pressure of a gas within an air cylinder, which is regulated by a switch valve of the gas supply. The switch valve is actuated by a switch signal that is directly output from the aforementioned control module to the valve. During downward movement of the mechanical assembly, it first accelerates to reach a velocity V1 at a time instant t1 and then moves constantly at a constant velocity equal to V1. When the mechanical assembly collides with the wafer stage on which a wafer is supported, the great impact from the collision may cause damages in the wafer and wafer stage.
Therefore, in order to protect the wafer and wafer stage from damages in the tight space inside the photolithography tool, critical requirements are placed on the safety and reliability of the wafer edge protection device. In particular, during a testing, if the wafer edge protection device is positioned improperly relative to the wafer stage, it is likely to exert a strong impact on the latter instantaneously when colliding therewith, as shown in FIG. 6a, in which the vertical axis A represents amplitude measured in hertz (Hz), and the horizontal axis t represents the time measured in second (s). A strong vibration signal generated from the collision indicates the instantaneous strong impact which may cause permanent damages in structure and performance of the wafer stage as well as vibration or even crushing of the wafer. While the impact from the wafer edge protection device on the wafer and wafer stage can be mitigated by reducing the vertical speed of the wafer edge protection device, a reduction in productivity will be resulted.
In another wafer edge protection device proposed in the prior art, a single power source is used to provide multiple synchronized forces by which a linear displacement is converted into multiple equal radial displacements. As a result, transfer of protective ring is accomplished with less power source consumption and a high degree of synchronization. However, this wafer edge protection device does not provide any protection against possible collisions during operation, leaving the wafer stage exposed to potential safety risks.